High voltage/current pulse generator using spark gaps

ABSTRACT

A high voltage/high current electric pulse generator. Spark gap switches (E1, E2, E3) having a trigger electrode are connected to a trigger control circuit (CD). Switch (E1) is connected between the outer conductor of cable (C2) and an inner conductor of cable (C1). Switch (E2) is connected between the outer conductor of cable (C3) and the inner conductor of cable (C2). Switch (E3) is connected between output terminal (S) and the internal conductor of coaxial cable (C3). The outer conductor of cable (C1) is connected to ground and voltage source (SHT) is connected to internal conductors of the cables. The source charges cables (C1, C2, C3) in parallel when switches (E1, E2, E3) are off. A first ballast resistor (r) is connected between the first electrode and the trigger electrode. Load resistor (R6) is connected between the output terminal(S) and the external conductor of cable (C3). Resistor (R5) is connected between the outer conductor of cable (C 3) and the outer conductor of cable (C2). Resistor (R4) is connected between the outer conductor of cable (C2) and ground. Resistor (R3) is connected between the inner conductor of cable (C3) and the inner conductor of cable (C2). Resistor (R2) is connected between the inner conductor of cable (C2) and the inner conductor of cable (C1). Resistor (R1) is connected between the inner conductor of cable (C1) and voltage source (SHT). Load resistors (R2, R3) are connected between the internal conductors of the coaxial cables.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a generator of very high voltage, andhigh current pulses.

2. Description of the Related Art

This invention is used in the supply of very high power klystrons andcertain electron tubes, such as free electron lasers. This invention canalso be used in the field of flash radiography or ionization. Thisinvention can also constitute a polarization source in acceleratorsusing collectors for recovering the energy of very intense electronbeans (from a hundred to a few thousand amperes).

In such applications, the requisite pulses are very high voltagerectangular pulses (above 500 kV), supplied under an impedance of 500 to3000 ohms and having the shortest possible rise time and the flattestpossible amplitude plateau, with a duration of 100 nanoseconds to a fewmicroseconds.

In existing generators, also known as modulators, use is generally madeof a low impedance formation line constituted by discreet components(capacitors and inductances). A high voltage thyratron controls thedischarge of the line. The operating voltage of the thyratrons does notexceed 60 kV, so that a step-up transformer has to be used. As a resultof the necessary electrical insulation associated by increasing the highvoltage, said transformer necessarily has significant magnetic leaks,which limits the rise; time of the output pulse. At 500 kV, the risetime is roughly one microsecond, which can only increase with the outputvoltage.

FR-A-2 638 304 discloses high voltage, pulse generators comprisingcoaxial cables charged in parallel by a high voltage source anddischarged in series across switches. If T is the transit time in acable with n cables and the charging voltage V, a voltage having anamplitude of nV/2 for duration 2T is obtained.

Although satisfactory in certain respects, these related art generatorssuffer from disadvantages. For example, their voltage is limited to afew dozen kilovolts and are also limited in current. This is, on the onehand due to the use of switches of the transistor, thyristor (andsimilar types) and, on the other hand, to the use of resistorspositioned between the output line and the earth or ground and which areexposed to the high voltage supplied.

SUMMARY OF THE INVENTION

The object of the present invention is to obviate these disadvantages byproposing a pulse generator able to operate at a very high voltage(several hundred kilovolts) and at a high current, i.e. at a very highpower. To this end, the generator according to the invention providesfor the use of spark gap switches and load and trigger resistors, whichdo not encounter see the very high voltage.

The invention specifically relates to an electric pulse generator inaccordance with FR-A-2 638 304 and which is characterized in that theswitches are spark gap switches having a first electrode connected tothe internal conductor of a cable and a second electrode connected tothe external conductor of the following cable and a trigger electrodeconnected to the trigger control circuit, two ballast resistors beingprovided between the first electrode and the trigger electrode and thesecond electrode and the trigger electrode and load resistors areprovided between the external conductors of the coaxial cables andbetween the internal conductors of said coaxial cables.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in greater detail hereinafter relative tonon-limitative embodiments and with reference to the attached drawings,wherein show:

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 A diagram of an embodiment of a generator according to theinvention.

FIG. 2 An example of pulses obtained by the generator according to theinvention.

FIG. 1 shows a generator having three commercially available coaxialcables C1, C2, C3, each having an external conductor Ce and an internalconductor Ci, three spark gap switches E1, E2, E3, a high voltage sourceSHT and a trigger circuit CD. The source SHT is connected to the cablesby load resistors R1, R2, R3, R4, R5, R6. The resistors R4, R5 and R6are positioned between the external conductors of the cables and theresistors R2, R3 between the internal conductors. The trigger circuit CDis connected to the trigger electrodes of the spark gap switches acrossa capacitor C and trigger resistors r1, r2, r3. Main supplementaryresistors r, r' balance the voltages between the main electrodes of thespark gap switches and the trigger electrode. The main electrodes of thespark gap switches are connected, on the one hand, to the internalconductor Ci of a cable and on the other hand, to the external conductorCe of the following cable.

The generator has a general output S connected to the external conductorof the final cable.

The operation of the device will now be discussed. The three cable C1,C2 and C3 are first charged in parallel by the high voltage source SHT.The control of the three spark gaps has the effect of connecting thethree cables in series, which will be discharged and provide a voltagepulse on the general output S. A single line charged to a voltage Usupplies, when it is discharged on a charge equal to its characteristicimpedance Z, a pulse of amplitude U/2 and of duration equal to doublethe propagation t/me in said line. If T is the propagation time and Nthe number of cables, as a result of the generator according to theinvention a pulse of duration 2T, amplitude NU/2 is obtained on a chargeof impedance NZ (N is equal to 3 in the illustrated variant).

For example, if 3 100 m coaxial cables are used with an impedance of 50ohms and charged to 50 kV, a pulse of 3×25=75 kV will be obtainedlasting one microsecond on a charge of 3×50=150 ohms. For N=20, 500 kVare obtained on 1000 ohms.

The high voltage supply SHT can be a commercially purchased having a 130kV, 5 mA supply. A trigger circuit should be chosen having a very shortduration pulses of approximately 10 ns.

All the components can be submerged in an oil tank, which ensures theelectrical response to the desired maximum voltage.

As the cables have a significant length (100 m or more), they can bewound onto a cylindrical support. Each cable is wound with its resistorsand spark gap switch. Thus, each stage has a modular character.

The operating frequency is limited by the heating due to the ohmiclosses in the cables and spark gap switches, the dielectric losses inthe cables and the oil, the convection movements induced in the oil bythe voltages applied and by the power of the continuous source. Theoperating speed can also be limited when protection is not provided byionizing rays being produced, if the charge is an electron gun.

FIG. 2 shows a pulse obtained by the generator of FIG. 1. The durationis close to one microsecond and the amplitude approximately 150 kV.

We claim:
 1. High voltage, high current electric pulse generatorcomprising coaxial cables (C1, C2, C3) each having an external conductorand an internal conductor, switch (E3) connected between an outputterminal (S) and the internal conductor of coaxial cable C3, switch (E2)connected between the outer conductor of cable (C3) and the innerconductor of cable (C2), switch (E1) connected between the outerconductor of cable (C2) and an inner conductor of cable (C1), the outerconductor of cable (C1) connected to ground a high voltage source (SHT)connected to all the internal conductors of the cables across resistors(R1, R2, R3, R4, R5, R6), said source charging all the cables (C1, C2,C3) in parallel when the switches (E1, E2, E3) are all off and a triggercontrol circuit (CD) connected to all the switches (E1, E2, E3) forturning the switches on and off wherein the switches are spark gapswitches (E1, E2, E3) having a trigger electrode connected to thetrigger control circuit (CD), a first ballast resistor (r) beingconnected between the first electrode and the trigger electrode and,wherein said load resistor (R6) is connected between the outputterminal(S) and the external conductor of cable (C3), resistor (R5) isconnected between the outer conductor of cable (C3) and the outerconductor of cable (C2), resistor (R4) is connected between the outerconductor of cable (C2) and ground, resistor (R3) connected between theinner conductor of cable (C3) and the inner conductor of cable (C2),resistor (R2) connected between the inner conductor of cable (C2) andthe inner conductor of cable (C1), resistor (R1) connected between theinner conductor of cable (C1) and the voltage source (SHT).
 2. Highvoltage/current electric pulse generator having a precharging period,when the generator charges, and a discharging period, when the generatoroutputs a high voltage/current pulse to an output terminal,comprising:coaxial cables each having an inner and an outer conductorincluding a first cable, an intermediate cable and a last cable; gapspark switches including a first switch, an intermediate switch and alast switch, wherein the first switch has a terminal connected to theinner conductor of said first cable and another terminal connected tothe outer conductor of said intermediate cable, wherein the intermediateswitch has a terminal connected to the inner conductor of saidintermediate cable and another terminal connected to the outer conductorof said last cable, wherein the last switch has a terminal connected tosaid output terminal and another terminal connected to the innerconductor of said last cable; a voltage source for precharging thecoaxial cables during said precharging period; a triggering circuit forswitching the gap spark switches simultaneously to an on and an offposition; and load resistors for precharging the coaxial cables duringsaid precharging period and not allowing a leakage current through saidload resistors during said discharging period, said load resistorsincluding a first resistor having a terminal connected to the voltagesource and another terminal connected to the inner conductor of saidfirst cable, a second resistor having a terminal connected to the innerconductor of said first cable and another terminal connected to theinner conductor of said intermediate cable, a third resistor having aterminal connected between the inner conductor of said intermediatecable and the inner conductor of said last cable, a fourth resistorhaving a terminal connected to ground and another terminal connected tothe outer conductor of said intermediate cable, a fifth resistor havinga terminal connected to the outer conductor of said intermediate cableand another conductor connected to the outer conductor of said lastcable, a sixth resistor having a terminal connected to the outerconductor of said last cable and another terminal connected to saidoutput terminal.
 3. The high voltage, high current pulse according toclaim 2, further comprising:a pair of first and second ballast resistorsconnected to a trigger electrode of the first switch said first ballastresistor having a terminal connected to the inner conductor of saidfirst cable and another terminal connected to the trigger electrode ofthe first switch, said second ballast resistor having a terminalconnected to the outer conductor of said intermediate cable and anotherterminal connected to the trigger electrode of the first switch.